ONETEP [1] is a DFT code which has been designed to achieve large (near-complete) basis set accuracy while retaining CPU and memory requirements which scale linearly with the number of atoms. Strictly localised orbitals (Non-Orthogonal Generalised Wannier Functions - NGWFs) are optimised in situ, expanded in a basis set of periodic sinc functions, which are mathematically equivalent to plane waves. Linear-scaling is achieved by taking advantage of the exponential decay of the Wannier functions, according to the physical principle of �near-sightedness of electronic matter� of Walter Kohn. Local and semi-local exchange correlation functionals are implemented in ONETEP based on algorithms that compute molecular integrals using localised three-dimensional Fast Fourier transforms (FFTs). In this talk we will describe our recent development for the calculation of Hartree-Fock exchange energy [2]. Two-electron repulsion integrals of NGWFs are needed for these calculations and due to their fully non-local nature we cannot use localised FFTs. We instead employ a resolution of the identity approach based on spherical waves. The calculation of Hartree-Fock exchange allows us to use hybrid exchange correlation functionals, which in many cases can provide more accurate molecular properties compared to local and semi-local variants. We will show examples validating our method against conventional cubic-scaling DFT codes, and demonstrating the linear-scaling of the computational time with the size. We will also outline our hybrid (MPI+OpenMP) parallelisation strategy for the code. The �two-electron integral engine� with NGWFs that we have developed for these calculations opens the door for the future development in ONETEP of ab initiowavefunction-based methods beyond Hartree-Fock with linear-scaling or reduced scaling computational cost.References 1. C.-K. Skylaris, P. D. Haynes, A. A. Mostofi and M. C. Payne, J. Chem. Phys. 122 , 084119 (2005).2. J. Dziedzic, Q. Hill, C.-K. Skylaris, J. Chem. Phys. 139 , 214103 (2013).

Linear-scaling Hartree-Fock exchange and hybrid exchange-correlation functionals with large basis set accuracy

248th National Meeting (2014)

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